The present invention relates to dispersants for use in fuels and in engine oil lubricants, especially for reducing soot-induced viscosity increase in heavy duty diesel engines.
Heavy duty diesel vehicles may use exhaust gas recirculation (EGR) engines in efforts to reduce environmental emissions. Among the consequences of recirculating the exhaust gas through the engine are different soot structures and increased viscosity of the oil at lower soot levels, compared with engines without EGR. It is desirable that oil exhibit minimal viscosity increase, e.g., less than 12 mm2/sec (cSt) at a soot loading of 6%.
It is also desirable that a lubricating oil composition maintain a relatively stable viscosity over a wide range of temperatures. Viscosity improvers are often used to reduce the extent of the decrease in viscosity as the temperature is raised or to reduce the extent of the increase in viscosity as the temperature is lowered, or both. Thus, a viscosity improver ameliorates the change of viscosity of an oil containing it with changes in temperature. The fluidity characteristics of the oil are improved.
Traditional dispersant viscosity modifiers (DVMs) made from ethylene-propylene copolymers that have been radically grafted with maleic anhydride and reacted with various amines have shown desirable performance to prevent oil thickening in diesel engines. Aromatic amines are said to show good performance in this regard. DVMs of this type are disclosed in, for instance, U.S. Pat. No. 6,107,257, Valcho et al., and U.S. Pat. No. 6,107,258, Esche et al., each Aug. 22, 2000, and U.S. Pat. No. 6,117,825, Liu et al., Sep. 12, 2000.
Other polymer backbones have also been used for preparing dispersants. For example, polymers derived from isobutylene and isoprene (“IOB/IP”) have been used in preparing dispersants and are reported in WO 01/98387. Also, nitrogen-containing esterified carboxyl-containing interpolymers prepared from maleic anhydride and styrene-containing polymers are known from U.S. Pat. No. 6,544,935, Vargo et al. The present invention provides a dispersant viscosity modifier based on a polymer of a vinyl aromatic monomer such as styrene and an ester copolymer such as esterified maleic anhydride monomer, distinguished from earlier dispersants by means of, among other things, the molecular weight and identity of the polymer backbone and, generally, the selection of the amine component, leading to a significantly different polymeric structure. The present materials exhibit superior performance in engine tests.
The present invention, therefore, solves the problem of providing a low cost dispersant viscosity modifier having improved performance in engine tests, providing a good viscosity index and good soot dispersion and toleration properties, particularly in diesel engine, and especially in heavy duty diesel engines employing exhaust gas recirculation. The materials of the present invention can further impart antioxidant performance properties to lubricants.